Wireless communication systems for cellular telephones in recent years must transmit many types of diverse information ranging from information with a comparatively small number of bits such as voice and written mail, to information with a large number of bits such as photos and movie files attached to mail and movie data for video telephony. To cope with this diversity, the system contains multiple bit rates and packet sizes to match the data quantity (number of bits) to be sent. Appropriate use of these bit rates and packet sizes increases the transmission efficiency. For example, the wireless communication standard cdma2000 1xEV-DO (Rev. A) established by the 3GPP2 standards project, contains twelve types of packet sizes over a wide range from 19.2 kbps to 1843.2 kbps for the reverse link traffic channel. The demodulation methods are a combination of BPSK, QPSK, or 8PSK modulation and Walsh coding, and a total of five modulation methods are used according to the packet size. FIG. 2 shows the combinations of packet size, modulation method, as well as component combinations required for demodulation. As can be seen in the table, there are two types of Walsh codes, namely W24 and W12. When the packet size is 4096 bits or less, either one of these code types can be utilized. When the packet size is 6144 bits or more, both types can be utilized.
The format for reverse link traffic channels for 1xEV-DO standards is described while referring to FIG. 1. Besides the data channel 101 used for transmitting the user data signal, the control channels such as the Pilot channel 102 and the RRI (Reverse Rate Indicator) channel 103 are CDMA (Code Division Multiple Access) multiplexed onto the same period in the packet of the reverse link traffic channel sent from the terminal. The Pilot channel 102 is utilized for synchronous tracking and for path search. The RRI channel 103 is used for sending an RRI signal to notify the base station of the data channel packet size sent by the terminal. The single subframe 104 is 6.66 milliseconds and the packets are sent in subframes. Also, Ack/Nack messages are exchanged by H-ARQ (Hybrid Automatic Repeat Requests) control between the terminal and base station after transmitting the packet on the reverse link on the data channel. If the base station did not receive the data channel packet Nack 105 is transmitted on the forward link traffic channel and the terminal re-sends the same data up to a maximum of three times. The base station stores the data during the retransmit period, accumulates the total demodulation results in the data which has been sent the previous time, and demodulates it again. However a single packet can only be sent every two sub-frames so that the data storage period at the base station when re-sending the data three times is 66.66 milliseconds (which is equal to 10 subframes) or more and therefore extremely long.
(1) In the 1xEV-DO standards, the RRI channel sends an RRI signal that is code multiplexed onto the data channel. Thus, the base station cannot know the packet size and modulation method until the RRI channel signal is decoded. In other words, the data channel cannot be decoded until the decoding of the RRI channel signal is completed.
(2) On the other hand, to implement H-ARQ control, the demodulated results on that data channel must be checked within one subframe, and the Ack/Nack must be sent on the forward link traffic channel. Thus, there are strict time limits on the data channel modulation.
(3) The required memory capacity of the base station is enlarged in order to store 10 subframes or more of data needed for H-ARQ control.
The above problem is not limited to 1xEV-DO standards but is a problem common in wireless communication standards using the CDMA system and standards that implement H-ARQ retransmission control such as 1xEV-DV standards. Such as when receiving the reverse link traffic channel as described, for example; however, the same concept applies to systems with the same type of RRI and retransmission problems on the forward link traffic channels, too.